Cellulosic ethanol refers to ethanol that has been produced from the cellulosic components of plants (i.e., cellulosic biomass) as opposed to ethanol produced from starches and sugars (e.g., corn ethanol). Cellulosic ethanol is increasingly being viewed as a preferred fuel alternative to starch or sugar ethanol because cellulosic parts of plants (e.g., corn stover) are generally non-edible and undesirable byproducts resulting from harvesting of edible crops, and thus, do not compete with food production. Typically, the cellulosic remnants are either discarded, used as fertilizer, or used as fodder. Furthermore, since cellulosic byproducts are not specifically produced as a fuel source, they do not require any additional expenditure for their production, and are therefore cheap and plentiful.
In a typical cellulosic ethanol process, raw cellulosic biomass material is pretreated in order to convert, or partially convert, cellulosic and hemicellulosic components into enzymatically hydrolyzable components (e.g., poly- and oligo-saccharides). The pretreatment process also serves to separate the cellulosic and hemicellulosic components from solid lignin components also present in the raw cellulosic material. The pretreatment process typically involves reacting the raw cellulosic biomass material, typically as a finely divided mixture or slurry in water, with an acid, such as sulfuric acid. Other common pretreatment processes include, for example, hot water treatment, wet oxidation, steam explosion, elevated temperature (e.g., boiling), alkali treatment and/or ammonia fiber explosion (see Mosier et. al., Bioresource Technology, 96 (2005) 673-686).
Typically, the pretreated biomass is then treated by a saccharification step in which poly- and oligo-saccharides are enzymatically hydrolyzed into simple sugars. The free sugars and/or oligosaccharides produced in the saccharification step are then subjected to fermentation conditions for the production of ethanol. Typically, fermentation is accomplished by combining one or more fermenting microorganisms with the produced sugars under conditions suitable for fermentation. The fermenting broth is typically heated, e.g., to 20-40° C. for yeast organisms (such as Saccharomyces cerevisiae), or to higher temperatures for thermophilic organisms (such as Thermoanaerobacter species). The ethanol is then removed (or partially removed) from the broth (e.g., by distillation), thereby leaving behind fermentation process water.
A significant problem encountered in the biomass-to-ethanol production process is the production of inhibitory compounds (e.g., acetate, furfural (and other aromatic aldehydes), ketones, and alcohols, such as hydroxy aromatics) during the pretreatment process of the raw cellulosic material. Inhibitor compounds have the deleterious effect of inhibiting one or more process steps (e.g., the saccharification or fermentation processes), thereby causing a decreased level of ethanol production. In particular, acetate is generally produced from hydrolysis of acetylated ferulates which are associated with the hemicellulose fraction of biomass.
Since copious amounts of water are used in the processing of cellulosic biomass materials, it is highly desirable to recycle used process water that has been separated from ethanol at the end of the fermentation process. However, since the process water contains inhibitory compounds, recycling process water to an earlier step has the effect of causing an accumulation (i.e., increasing concentration) of inhibitory compounds used in the process. Eventually, the concentration of inhibitory compounds can be high enough to reduce efficiency of the process to a level that renders the process unfeasible (e.g., at or below 10% yields of ethanol).
The accumulation of inhibitor compounds is compounded and accelerated by the general practice of using a high solids loading (i.e., generally at least 20% solids loading) of raw biomass in the process. Both recycling and high solids loading can dramatically lower ethanol yields. For example, for a high solids loading of 25%, an increase in water recycling from 10% to 25% can result in a reduction in ethanol yield from 65% to 5% (Schell, DOE OBP Biochemical Processing Integration, Biochemical Platform Review Meeting, Denver, Colo., Aug. 7-9, 2007).
The accumulation of inhibitory compounds caused by recycling discourages recycling and conservation of process water, and encourages disposal of contaminated water (often into the environment) and replacement with fresh water. If water recycling is pursued, the methods for removing inhibitors are generally costly and environmentally unfriendly by requiring the use of non-renewable fuels. Some examples of currently practiced inhibitor-removal processes include, for example, separation methods (e.g., by use of specialty membranes or ion exchange membranes), and anaerobic digestion or microbial gasification (e.g., methanation).
Accordingly, there would be a benefit in a cost efficient method that removes inhibitor compounds from process water used in a cellulosic biomass-to-ethanol process. There would be a particular benefit in such a method in which significant cost savings results from use of renewable energy technology. Such a method would beneficially promote conservation of water by allowing ethanol fermentation wastewater to be recycled, while at the same time maintaining high ethanol yields in a continuously recycled system. In addition, such a method would allow a portion of the process water that is not recycled to be safely discharged into the environment.